A study was made of clinical and pathological features of patients with active infective endocarditis (IE) of the aortic valve, and a comparison was made of these findings in patients with previously normal versus previously stenotic aortic valves. The results obtained show that active IE superimposed on a stenotic aortic valve differs in some features compared to active IE occurring on a nonstenotic aortic valve. Because ring abcess is so common when active IE involves a stenotic aortic valve in adults, operative intervention at an early stage may be warranted in such patients.